1. Field of the Invention
The invention pertains to the art of micro-electro-mechanical modulation systems and, more articularly to electrostatic voltmeters.
2. Description of Related Art
In an effort to achieve reliable, low cost, and potentially high precision micro sensors, development continues in the technology of integrating small mechanical elements onto silicon substrates. Polysilicon microbridges have been driven vertically and laterally as resonant microsensors. With respect to laterally driven microbridges, short displacements of a comb type drive of the type shown and described in U.S. Pat. No. 5,025,346, typically on the order of one to ten micrometers, lead to very weak sensed signals. The disclosure of the U.S. Pat. No. 5,025,346 patent is incorporated herein by reference.
One such class of devices hampered by the weak output signals is electrostatic voltmeters (ESV's). Non-contacting ESV's are utilized, for example, within xerographic printers to measure the surface voltage of the photoreceptor; however, additional applications are known in the art. In almost all non-contacting voltmeters the measurement of the surface voltage is made using capacitive coupling of a sense probe to the measured surface. This capacitive coupling is typically modulated by a mechanical shutter between the sense probe and the measured surface. The output current of a capacitively coupled sense probe is given by: EQU i.sub.s (t)=dQ(t)dt=d/dt[C(t)]V.sub.s (t)]=C(t)dV.sub.s (t)/dt+V.sub.s dC(t)/dt, EQ.1
where Q(t) is the time-varying charge induced on the sense probe, C(t) is the time-varying capacitance and V.sub.s (t) is the time-varying measured surface voltage. For the measurement of the d.c. component of the photoreceptor voltage, V.sub.dc, the equation can be simplified to: EQU i.sub.s (t)=V.sub.dc dC(t)/dt=V.sub.dc [.differential.C(x,t)/.differential.x ][.differential.x/.differential.t], EQ.2
where the shutter position determines C(t) at any time. For a sinusoidal drive signal the capacitance is given by: EQU C(t)=C.sub.o +C.sub.m sin(.omega.t). EQ.3
Here C.sub.O, is the d.c. component of the capacitance that does not change with time and C.sub.m is the a.c. component of the capacitance that changes with the shutter motion. To maximize the signal, either the change in the capacitance with respect to shutter position (i.e. .differential.C(x,t)/.differential.x, from EQ. 2) or the shutter velocity (i.e. .differential.x/.differential.t, from EQ. 2), or both, can be increased.
Since, the shutter is typically operated at resonance, the resonant frequency determines the shutter velocity, .differential.x/.differential.t. On the other hand, shutter geometry determines the change in capacitance with respect to shutter position. Previous micro-electro-mechanical ESV's employ a simple shutter that masks a portion of the sense probe. The shutter is actuated by an electrostatic comb type drive, which typically has a maximum displacement, .delta.x, on the order of 1-10 .mu.m.
In such a design the modulated change in the capacitance is determined by the permittivity of freespace .epsilon..sub.o, and the area A: EQU C(t)=C.sub.o +(.epsilon..sub.o.delta.A/d)sin(.omega.t)=C.sub.o +(.epsilon..sub.o.lambda..delta.x/d)sin(.omega.t), EQ.4
where .delta.x is the displacement of the shutter, and .lambda. is the length of the shutter, for a spacing d between the sense probe and the photoreceptor. .delta..sub..chi. is limited by the displacement of the comb drive. Thus, to increase the modulated area the length of the shutter, .lambda., can be increased. However, increasing the length of the shutter, .lambda., will correspondingly increase the size and mass of the resonator, leading to a decrease in sensor spatial resolution and a lower mechanical resonant frequency. This has the undesirable effect of reducing sense probe current output.
Thus, an alternative solution has been sought that provides a larger effective modulation area in order to obtain a stronger signal than previous designs.